The relationship between cumulative residential exposure to radon and cancer risk is being evaluated in an ongoing collaborative study involving Yale University and the University of Utah. The study is designed to evaluate the potential interaction between radon and cigarette smoke exposure. Approximately half of the expected 1000 smokers and 750 nonsmokers with lung cancer and 2100 population controls from Connecticut, Utah, and Southern Idaho have been enrolled. Detailed residential and exposure histories are being obtained, and radon measurements are being made in past homes of participants using year-long alpha track etch detectors, in order to estimate cumulative radon exposure since birth or age 25 for each subject. Data collection has been completed in a pilot phase intended to evaluate factors such as housing characteristics and geology that "predict" radon levels in homes. These data will be analyzed to develop strategies for dealing with missing radon measurements, an important problem in studies involving lifetime exposure assessment. A companion study in Connecticut involving 125 children with cancer and 125 cancer-free controls designed to evaluate the potential childhood cancer risk associated with residential radon exposure is nearly complete. In a study of childhood leukemia risk associated with residential radon exposure that is being conducted in collaboration with a cooperative childhood cancer treatment group, 250 of 325 leukemia patients or controls from a larger case-control study of risk factors for childhood leukemia were found to be eligible for radon measurement based on residential history. Interviews have been completed and detectors placed for 140 of the eligible subjects.